Most MEMS sensors are built onto silicon based wafers of approximately 500 μm thickness. While these thick substrates confer stability during fabrication and over the long term, the thickness limits applications in tight spaces, which includes many biomedical and industrial conditions. The rigidity and biocompatibility of silicon based sensors are additional limiting factors. Overcoming these issues is particularly challenging for diaphragm based sensors, due to the tight control required to build three-dimensional cavities and diaphragms at such a small scale.
The active region of many silicon based sensors is the deflecting diaphragm near the surface of the sensor. Typically, the active region ranges from the low-micron to sub-micron scale, which is a small fraction of the overall sensor thickness. The sensor profile can be significantly reduced if the inactive substrate is replaced with a thinner substrate or if the active region is integrated into the device package.
Passive ultrasonic sensors, methods and systems for their use are described in U.S. Pat. No. 6,770,032. Specifically, passive acoustic sensors having at least two flat parallel acoustically reflecting surfaces. At least one reflecting surface is on a member which is movable such that the distance between the reflecting surfaces varies as a function of a physical variable to be determined. Preferably, the sensor is made such that the intensity of a first portion of incident acoustic waves which is reflected from one reflecting surface is equal or substantially similar to the intensity of a second portion of the incident acoustic waves which is reflected from the other reflecting surface. The first portion and the second portion interfere to form a returning acoustic signal having one or more maximally attenuated frequencies which is correlated with the value of the physical variable. The internal acoustic signal is received and processed to determine the value of the physical variable from one or more of the maximal attenuation frequencies. Methods and systems for using the passive sensors are disclosed.
Existing systems such as those described above use an ultrasound probe, with a limited transmitting/receiving bandwidth, which permitted limited sensing of resonators, because most feasible mechanical resonators have natural frequencies in the audible or just above audible range under physiologically relevant pressure.
A passive sensor system using ultrasonic energy is also described in PCT Patent Publication WO 1995020769. In particular, a passive sensor system (14) utilizing ultrasonic energy is disclosed. The passive sensor system includes at least one ultrasonically vibratable sensor (10) and an ultrasonic activation and detection system (20, 22, 24, 25). The sensor (10) has at least one vibration frequency which is a function of a physical variable to be sensed. The ultrasonic activation and detection system (20, 22, 24, 25) excites the sensor and detects the vibration frequency from which it determines a value of the physical variable. The sensor includes (see FIG. 2-4) a housing, a membrane which is attached to the housing and which is responsive to the physical variable, a vibratable beam attached to the housing at one end and a coupler, attached to the membrane and to a small portion of the vibratable beam, which bends the vibratable beam in response to movement of the membrane.
The ability to measure pressure locally can be used in the analysis of certain conditions. Diabetics are prone to foot ulceration, with a population prevalence of approximately 8% and a lifetime risk of up to 25% (Margolis, Boulton). Loss of innervation due to diabetic peripheral neuropathy induces muscle laxity and associated skeleton deformities, as well as loss of sensation. This increased risk of focal stress points and reduced ability to accommodate to the initiating trauma greatly contribute to the formation of ulcers, which can progress in severity to the point where amputation is necessary. Critically, prevention and management by proper monitoring of foot conditions could reduce amputations by 50% (Driver).
Treatment of an ulcer is difficult after formation due to repetitive damage and compromised healing in diabetics. Over a third of the direct expenditure on diabetes in the US ($116 billion) is on ulcer treatment, with each treatment costing on average $28,000. Prevention by careful monitoring of the condition of the feet is considered to be the best approach and is thought to potentially avert half of the amputations due to ulceration (Driver).
Space constrictions limit conventional sensing devices in many environments, such as in shoes or insoles. Wires, power supplies, circuitry, and antennas in conventional approaches are all too large and cumbersome to fit without disruption. Electromagnetic resonance sensing offers a solution because these simple wireless systems only require a coil and a capacitor to operate. As such, they can be made small enough to wirelessly sense otherwise inaccessible environments. They are interrogated wirelessly by magnetic coupling. In some cases, the resonant system can entirely replace the conventional radio link system; in other cases, it can be used together with a radio link to extend the sensing range. The mechanism of resonance sensing is not widely used or known, probably because most sensing environments are accessible via wired sensors. Presently, to synthesize this mechanism of sensing with an application in foot pressure sensing requires a breadth of knowledge in a numerous disparate fields, including physics, mechanics, electrical engineering, and clinical medicine.
Peripheral neuropathy contributes to the high prevalence of foot ulceration in diabetics. Several systems, integrated into shoe insoles and socks, are currently available for monitoring foot pressures to prevent ulceration. However, these systems have practical limitations and inconveniences for end user, such as dangling wires or tenuous electronics.
Embodiments of the present disclosure offer a clean solution through a resonant wireless system in a shoe insole. The sensing insole is physically simple and durable, requires no on-site power supply or circuitry, and can wirelessly transmit pressure signals to a nearby device with radio link capability, such as a clip on the outer shoe, an anklet, or a waist belt. To our knowledge, no resonant wireless sensing system has been applied to measuring foot pressures in the patent or scientific literatures. An embodiment has been enabled with a thin film capacitive pressure transducer which demonstrates functionality and excellent pressure sensitivity.